104 THE FRESH-WATER LOCHS OF SCOTLAND 
Classification of Lakes 
It is customary to divide fresh- water lakes into three classes, depend- 
ing on the change of density of water with temperature. For con- 
venience of reference the following table gives the density of pure water 
at intervals from the freezing point up to 70° Fahr., warmer than 
which it is rare to find water in Scottish lakes : — 
Temperature. Density. 
32° Fahr. 0-99987 
39 1-00000 
45 0-99992 
50 0-99973 
55 0-99943 
60 0-99904 
65 0-99857 
70 0-99800 
It is seen from the table that water has a maximum density point 
about 39° Fahr., and it is this fact which renders possible the freezing 
of the surface of lakes. Fresh-water lakes are classed as : (1 ) those 
whose waters never fall below 39° Fahr., (2) those whose waters always 
have a temperature lower than 39° Fahr., and (3) those whose waters 
are sometimes above and sometimes below 39° Fahr. The names given 
to these classes respectively — tropical, polar, and temperate — are not 
very appropriate, suggesting as they do that one class of lakes belongs 
to the tropics, another to the polar regions, and the third to the 
temperate zone. This is not so, but the names have now obtained 
such general recognition that it is not advisable to discard them. 
In Scotland there are no lakes of the polar type or class. In a 
lake of this class the temperature of the water never rises above the 
maximum density point. As a consequence of this the coldest water 
is always at the top, the lower layers having a higher temperature ; 
at the bottom the water may have a temperature of 39° Fahr., but 
no higher — otherwise the lake would not be of the polar class. Such 
lakes can only be found in the polar regions or at great altitudes, and 
in them the cycle of temperature changes is the reverse of what is found 
in tropical lakes. It will be explained later how in tropical lakes 
the temperature of the water is nearly uniform in winter, and how 
stratification sets in as the summer advances. But in polar lakes it 
is in summer that a uniform temperature is found throughout the 
lake. As winter draws on the lake becomes thermally stratified, as 
will be seen by reference to ForePs diagrams (figs. 37-39), which are 
reproduced by permission, p^^ p^, . - ■ are depth measurements ; 
the horizontal scale measures time ; m^^ m^, . . . n^^ n.^ . . . are 
isothermals showing the depth at which the temperatures of mg, 
. . . 7^2? • • • are to be found at any period of the year. The 
isothermal for the maximum density point is represented by a double 
